Given the concentration of AB/DLs on the internet, not just ADISC but other sites too, it does feel like there are a lot of us on this planet. However, there are many other factors to take into consideration. Considering the population of the Earth is over 7 billion, even a seemingly small percentage would mean a lot of AB/DLs on this planet. One percent of this population is over 70 million people. That doesn't mean there are very few of us; there are certainly more of us than anybody would think. It's all relative to the scope.
You also need to take personality factors into account. On this site, there are DLs, there are AB/DLs, there are AB/DL supporters, there are normal people who are curious about AB/DL, and there are many other groups of people I have not/forgot mentioned on this site. AB/DL is one huge umbrella to which many people fall under and branch out.
Finally, there are the AB/DLs who remain in the closet for various reasons. I have been practising DL since middle school, but I did not post or say anything DL-related on the internet until earlier this year, mainly because of social stigma. When you calculate the number of AB/DLs on this planet, you need to add some percentage error to your total to compensate for those still in the closet. So to summarise, there can either be a lot of us or few of us; it is all relative to the size of the scope you look at the issue through.